PROJECT SUMMARY/ABSTRACT. The intestinal epithelium is the first line of defense against the trillions of bacteria in the intestinal lumen. Failure to prevent or limit bacterial invasion can lead to infection and may trigger a harmful inflammatory response, as seen in inflammatory bowel disease. Intraepithelial lymphocytes (IEL) expressing the ?? T cell receptor rapidly respond to bacterial translocation through modulation of their migratory behavior and release of soluble host defense factors, including cytokines and antimicrobial peptides (AMP). Though ?? IELs are an essential part of host antibacterial response, many of the processes surrounding ?? IEL-mediated host defense remain unclear. The objective of this application is to elucidate the mechanisms through which ?? IELs contribute to host antibacterial defense. Preliminary data suggests that ?? IEL surveillance behavior and effector function may be partially mediated by cell-autonomous recognition of microbe-associated molecular patterns (MAMP) through Toll-like receptor/MyD88 signaling. Moreover, a newly-identified requirement for ?? IELs in lipopolysaccharide (LPS)-induced epithelial cell shedding may represent a novel form of ?? IEL- mediated host antibacterial defense, as extrusion of enterocytes infected with intracellular bacteria limits pathogen proliferation within the epithelium. These observations have led to the central hypothesis that ?? IELs contribute to host defense against bacterial invasion by activating migratory and effector responses following cell-autonomous MAMP recognition and by promoting the shedding of infected enterocytes. The aims of this application are to 1) determine the contributions of ?? IEL MyD88 signaling to host defense against acute bacterial invasion and 2) determine the mechanisms through which ?? IELs regulate cell shedding in response to acute bacterial challenge. The role of ?? IEL MyD88 in mediating MAMP-induced changes in ?? IEL migration and cytokine/AMP secretion will be assessed using inducible, ?? T-cell-specific MyD88 knockout mice and a combination of in vitro and in vivo approaches. Further, the contribution of ?? IEL MyD88 to antibacterial defense will be determined using Salmonella Typhimurium infections. Next, the mechanisms by which ?? T cells promote LPS-induced epithelial cell shedding will be interrogated by quantifying cell shedding in mice exhibiting altered ?? IEL migratory phenotypes or following ?? TCR inhibition. The requirement for ?? IELs in shedding of Salmonella-infected enterocytes will be assessed at early infection timepoints. Completion of these aims will provide novel insight into the mechanisms driving ?? IEL-mediated antibacterial defense. The results of these studies will contribute towards the long-term goal of determining the therapeutic potential of targeting ?? IEL effector functions as a means to prevent excessive inflammation by promoting rapid clearance of infection. The proposed research and training plan, along with my mentors, advisory committee, and the interdisciplinary environment at my home institution, will help me achieve my long- term career goal of becoming an effective physician-scientist involved in independent translational research.